Cathode having a cavity in the emissive element

ABSTRACT

A cavity is provided in a wire of a directly-heated cathode so that the temperature at the area of the cavity will be higher when a current is passed through the wire than on either side of the cavity. As a result, the cavity acts as a limited emissive surface and the cathode obtains a longer service life and a more stable construction.

0 United States Patent 1 [l 11 3,755,705 Zwanenburg Aug. 28, 1973 CATHODE HAVING A CAVITY IN THE [56] References Cited EMISSIVE ELEMENT UNITED STATES PATENTS [75] Inventor: Gooitzen Zwanenburg, Emmasingei, 1,749,780 3/ I930 Rentschler 313/336 X Eindhoven, Netherlands 3,462,635 8/1969 Broers 3l3/336 x [73] Assignee: Philips Corporation, New York, Primary Examiner David Schonberg Assistant Examiner-Paul A. Sacher [22] Filed: Jan. 31, 1972 Attorney-Frank R. Trifari [21] App]. No.: 222,149

[57] ABSTRACT A cavity is provided in a wire of a directly-heated cath- [30] Forelgn Apphcanon Pnonty Data ode so that the temperature at the area of the cavity Feb. 13, 197] Netherlands 7101954 be when a Current is passed through the wire than on either side of the cavity. As a result, the cavity [52] US. Cl. 313/341 acts as a limited emissive surface and the cathode g glg g gi tains a longer service life and a more stable construc- Hon.

1 Claim, 4 Drawing Figures CATHODE HAVING A CAVITY IN THE EMISSIVE ELEMENT The invention relates to a cathode having an emissive element to be heated by passage of an electrical current for the emission of thermal electrons.

Cathodes of this kind comprise an emissive element, preferably made of a wire or a strip of tungsten or a similar refractory material, which can be heated by current passage to a high temperature such that electron emission occurs. These so-termed directly-heated cathodes are used in particular in electron microscopes, electron beam machining apparatus and the like, and have the advantage that they are comparatively well capable of withstanding residual gases, and that they can be exposed, also after use, to the atmosphere without their emissive properties being adversely affected.

The invention has for its object to provide a cathode of the kind set forth in which a high emissive-current density is possible due to a favourable heat distribution on the emissive surface, the emissive surface being constructively limited. A cathode of the kind set forth is characterized in that the emissive element comprises a cavity which reduces the cross-sectional area from the current passage for the emissive element.

As the emissive element in a cathode according to the invention has a smaller section, viewed crosssectionally, at the area of the cavity than in adjoining portions, the wire at the area of a cavity will be heated to a higher temperature when a current is passed through than will the adjoining portions. As the surface of the emissive element in the cavity will emit less heat by radiation than the remaining surface, the temperature difference between the cavity and adjoining portions will be increased. Consequently, a high emission can occur in the cavity without electron emission from the edges of the cavity and the remaining surface of the emissive element. The edge of the cavity, consequently, forms a limitation of the emissive surface of the emissive element.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIGS. la through lc show an emissive element in the form of a circular-cylindrical wire, provided with a cavity according to the invention,

FIG. 2 shows a cathode-grid unit having an emissive element according to the invention.

FIG. lb is an axial sectional view through a round wire 1 which is made, for example, of tungsten and which has a diameter of, for example, 200 microns. The wire 1 may also have a rectangular or other section, and may also be made of titanium, lanthanum boride or an other material known to be used for directly-heated cathodes. The wire 1 has a cavity 2 with an edge 3 and a limiting surface 4. At the surface of the wire 1 the cavity 2 has a transverse dimension of, for example, I25 microns and a depth with respect to this surface of, for example, again 125 microns. A heating current for the wire 1 is denoted by arrows 5 and 6.

FIG. 1a is a front view of the wire I, viewed from a direction from which a control grid is to be provided in the vicinity of the wire. The edge 3 of the cavity thus viewed is circular in this example, but cavities having an other limitation, such as an ellipse, may also be used. The area 4, i.e., the surface in the cavity 2, then acts as the emissive surface, whilst the remaining surface 7 of the wire remains below the emissive temperature during operation.

FIG. 1c shows a cross-section 8 of the wire 1 through the centre of the cavity. A passage surface 9 for the heating current is reduced at that area by the dimension 10 of the cavity in this sectional plane. As the surface 4 in the cavity assumes a higher temperature during operation than the remaining surface 7 of the wire, the cavity will become ever larger, due to evaporation of material, and after some time it will be limited, for example, by the limitation denoted by the broken line 1 1. During and because of such cavity enlargement, the passage surface 9 of the heating current decreases further yet, and the temperature and hence the electron emission at this area will increase, the intensity of the heating current remaining the same. Due to the increase of the cavity and hence of the limiting surface, the surface contributing to the emission increases, and hence for a given temperature of the cathode so does the overall emission. Conversely, the current intensity of the heating current can be reduced after a given period of operation. In prior art directly heated cathodes the emissive surface will decrease due to evaporation of the wire at all sides, and the temperature of the wire has to be increased to ensure constant electron emission, which results in quicker evaporation again. As evaportion of any important extent occurs in the cathode of the present invention only in the cavity in the cathode and not, as in the prior art cathodes, over the entire wire surface, substantially less material will be dispersed in the electron beam apparatus in which the cathode is used. As result, less contamination occurs in apparatus provided with a cathode according to the invention, so that a prolonged, reliable performance is achieved. As the wire portions adjoining the cavity do not evaporate, more stable positioning of the wire is possible.

The edge 3 of the cavity will have to be properly centred with respect to an aperture in a control grid when the cathode is used in an electron beam apparatus. Consequently, it may be advantageous to use the control grid as the control element when the cavity is provided. A practical solution is to make the cavity through the control grid aperture. This can be readily realized for many methods of providing the cavity, such as spark-erosion, electron beam machining, photographic etching and the like. The cathode and the control grid then form a constructional unit and can be built into apparatus as a cathode-grid unit. This method of providing the cavity in 'the wire is particularly advantageous if use is made of non-rotation-symmetrical grid bore-holes having a limitation of the emissive surface which is adapted thereto from an electron-optical point of view, i.e., in this case the cavity limitation on the surface of the wire.

FIG. 2 is a diagrammatic view of a cathode-grid unit according to the invention. The cavity 2 is situated at a point 12 of the wire 1 which is bent at this area. A grid bush 13 is mounted about the wire by means of, for example, thermally and electrically insulating supports 14 and 15. These supports may be made, for example, of ceramic material. A grid plate 17, provided with a grid aperture 16, closes the grid bush opposite the wire tip. When the cavity is provided, the spark gap can thus be determined by the grid aperture in the case of spark erosion, the grid aperture can be used as the control electrode in the case of electron beam machining and exposure can be effected through the grid aperture in the case of photographic etching.

lf difficulties are experienced during operation from electrons which are emitted from the surface 4 but near the edge 3, a receding cavity may be provided. This is denoted in FIG. 1c by the dotted line 11.

Cathodes according to the invention can be used in particular in electron beam apparatus in which a high emissive current density is desired. in addition to the high current density in the centre of the emitted electron beam, they also supply, due to the limitation, a comparatively small, non-usable, edge current and make only a comparatively small contribution to the centered with respect to said grid aperture.

# t I i 

1. An electron beam apparatus comprising: a. a directly heated cathode having a wire-shaped emissive element for emitting thermal electrons, said element comprising a cavity that locally reduces the cross section of said element to a size smaller than the cross-section of said element at portions adjoining said cavity, and b. an apertured control grid located opposite said emissive element, said cavity being substantially centered with respect to said grid aperture. 